Someone helped me in my thread about this so I'll pass along the info:
400 PP - just use the stock Opel Speedster.
500 PP - upgrade a Lotus Elise 111R (in the new dealerships)
650 PP - get a 2J race car and use the power limiter to lower it to 650 PP.
You can lower the ride height a little and increase the spring rates a little for all the cars if you'd like. Also lower the top speed in the transmission to like 140-150. Maybe increase the camber to like 0.3 if you're on a track that has a lot of turns and not a lot of straightaways.
The 500 PP races with the Elise are the hardest out of the 3. The other two should be very easy to win with those vehicles.